1. The Field of the Invention
The present invention pertains to the formation of a vent orifice in fluid catalytic cracking direct-connected cyclone apparatus.
2. The Prior Art
Patent application Ser. No. 07/811,729, filed Dec. 23, 1991, now U.S. Pat. No. 5,248,411, the disclosure of which is incorporated herein by reference, describes an apparatus for rapidly separating catalyst from a cracked hydrocarbon gas in a fluidized catalytic cracking (FCC) unit. It also describes a process for withdrawing stripper gas from an FCC reactor vessel. A unique feature of the described invention is the location of the vent orifice for reactor and stripper gasses in an annular space formed around the reactor cyclone outlet tube and the roof of the reactor cyclone. This location for the vent orifice has been shown to have a unique advantage over systems practiced by others in the operation and pressure balance of direct-connected cyclone systems.
In the direct-connected cyclone system the vent orifice may serve as an expansion connection between the first stage reactor cyclone and the second stage or upper cyclone. In this case, freedom of movement between the reactor cyclone outlet tube and cyclone roof must be maintained. Sizing of the vent orifice and maintenance of the vent orifice clearance is critical to maintaining proper operation of the direct-connected cyclone during an extended run on an FCCU. The tolerances required to maintain the proper pressure balance in the system are very small. For example, in one system, the width of the annular gap of the vent orifice is only about 20 mm. In some designs it may be necessary to provide for lateral expansion which exceeds the width of the gap between the cyclone outlet tube and cyclone roof. In other words, when the gap is designed for the proper pressure drop, there may not be enough tolerance or clearance to provide for lateral expansion.
The process of fluid catalytic cracking (FCC) comprises mixing hot regenerated catalyst with a hydrocarbon feedstock in a transfer line riser reactor under catalytic cracking reaction conditions. The feedstock is cracked to yield gasoline boiling range hydrocarbon as well as degradation products, such as coke which deposits on the catalyst causing a reduction in catalytic activity. Hydrocarbon vapor and coked catalyst are passed from the top of the riser reactor directly to a separator vessel, typically a cyclone separator, wherein catalyst is separated from hydrocarbon. In the FCC art, the separator vessel is termed the reactor vessel. The separated catalyst is passed to a stripper wherein it is contacted with a stripping gas to remove volatile hydrocarbon. Stripped catalyst is then passed to a separate regeneration vessel wherein coke is removed from the catalyst by oxidation at a controlled rate. Catalyst, substantially freed of coke, is collected in a vertically oriented regenerated catalyst standpipe. The catalyst is passed from the standpipe to the riser reactor for cyclic reuse in the process.
U.S. Pat. Nos. 4,623,446 and 4,737,346 to J. H. Haddad et al teach a closed coupled cyclone separator system in the reactor vessel of a fluid catalytic cracking apparatus. Means is provided for blending stripping gas with cracked hydrocarbon as it flows to a directly coupled riser cyclone separator. As shown in FIGS. 7 and 8, the riser reactor conduit is modified to comprise an overlapping downstream portion 118 to provide an annulus between the upstream portion 117 and the downstream portion 118. The annulus is covered by a flat metal ring having orifices 125 in open communication with the reactor vessel, enabling stripping gas to pass into the downstream conduit 118. A riser cyclone dipleg is sized, as seen in FIG. 5, to admit at least a portion of stripping gas from the stripping zone to pass countercurrent to catalyst passing downwardly through the dipleg.
U.S. Pat. No. 4,502,947 to Haddad et al discloses a closed cyclone fluid catalytic cracking catalyst separation method and apparatus. In the closed cyclone, hydrocarbon product and catalyst are passed directly into a cyclone separator from a riser without passing into the atmosphere of the reactor vessel. Avoiding the atmosphere of the reactor vessel reduces both excess catalytic cracking and high temperature thermal cracking.